Device for non-invasively detecting the oxygen metabolism in tissues

ABSTRACT

The invention relates to an apparatus for noninvasive determination of the oxygen turnover and data derived therefrom with an optical sensor (S) to be placed on the tissue with one or more light sources (W, L) which send light through the optical fibers to the sensor (S), one or more detectors (DD, DR) which receive light backscattered by the tissue through optical fibers, and an evaluation unit.

DESCRIPTION

The invention relates to the determination of the local oxygen turnover,the oxygen consumption, the oxygen content and the locally transportedamount of oxygen in perfused tissues using an optical sensor as setforth in the preamble to claim 1.

Oxygen is of elemental importance for almost all cells in biologicaltissues. In perfused tissues, oxygen is mostly transported in a formbound to hemoglobin, which is incorporated in the erythrocytes, from thelung to the oxygen-consuming cells. Arterial hemoglobin in the systemiccirculation which has undergone saturation in the lung is almost 100%saturated with oxygen. In the peripheral tissue, the oxygen is thendelivered along the capillaries to the cells. Correspondingly lowersaturations of hemoglobin with oxygen are measured in the tissue regionsat the venous end of the capillary and in the downstream venules andveins.

Noninvasive measurement of the oxygen content in tissue according to theprior art is possible only with very elaborate methods such as NMR(nuclear magnetic resonance). In addition, dynamic measurements of theoxygen consumption or monitoring of the oxygen removal through tissuerespiration are not as yet possible with NMR methods.

The energy metabolism of the cells, which is closely linked to theoxygen uptake, can be determined only invasively, by taking samples oftissue or body fluids followed by biochemical laboratory analysis of themetabolic products. Thus, no dynamic measurements of the oxygenconsumption or long-term monitoring are possible with these biochemicallaboratory methods.

The arterial saturation of hemoglobin SO₂ (%) in the arterial system canbe determined by so-called pulse oximeters, but these do not customarilyallow the capillary-venous hemoglobin saturation levels, the hemoglobinconcentration in the measured volume or the blood flow to be determined.The methods used in pulse oximetry are based on findings from cuvettephotometry and use only a few individual wavelengths, so that a changein the measured volume and the recording of changes in scattering orabsorption cannot be taken into account. This means that thesemeasurement methods are associated with considerable measurementuncertainties. The advantage of these instruments to date is that theyare very convenient and easy to handle.

Some spectrometric and spectroscopic methods are now known fordetermining the saturation of hemoglobin SO₂ [%] in the capillary venoussystem and the amount of hemoglobin in the measured volume (EMPHO,NIRO500, HemoSpec and newly AbTisSpec). These systems in some casespermit quantitative determination of the saturation of the hemoglobinwith oxygen SO₂ [%]. By comparison therewith, determination of theamount of blood or the hemoglobin concentration in the capillary venoustissue bed is possible with said systems only in relative numericalmeasures. It is not possible with said methods to make a quantitativestatement about the hemoglobin concentration in the measured volume orthe amount of blood in the tissue, because these methods lack a relationto the detection region and thus to the light path, and lack depthselectivity.

Two methods offer a possible approach to quantifying the light path andthus the effective measured volume:

-   firstly so-called PMS (phase-modulated spectroscopy) and secondly    TRS (time-resolved spectroscopy). These time-resolved methods have    the crucial disadvantage, however, that they can operate with only a    few wavelengths because of the need to use laser sources. It is thus    possible to determine the saturation of hemoglobin with O₂, which is    found from the change in spectral light absorption, only with    difficulty and then only with very costly laser methods.

Laser Doppler methods such as, for example, the OptoFlow can be used toobtain the blood flow rates and the amount of blood flowing as relativemagnitudes. This class of optical methods based on evaluation of Dopplersignals does not make it possible to gain information for determiningthe oxygen loading of the erythrocytes (the red blood corpuscles) or thecurrent measured volume.

However, for a complete description of the state of oxygen supply inperfused tissues, it is necessary in addition to the parameter of bloodflow rate also to determine by measurement techniques the amount ofblood moved (number of red blood corpuscles moved), the total amount ofblood in the tissue (also called the hemoglobin concentration) and theloading of the red blood corpuscles with oxygen (also called hemoglobinoxygenation or saturation SO₂).

EP 771 546 A2 discloses an apparatus for the noninvasive opticalmeasurement of the blood flow in biological tissue.

The propagation of light in biological media, scattering suspensions,tissue sections and intact tissues and cell structures is determined bytheir optical properties. The propagation of light is moreoverdetermined by the two basic phenomena of light absorption and lightscattering. The absorption of light or the attenuation in the sense ofconverting the light energy into light of a higher wavelength takesplace through interaction with cellular and subcellular structures ofmacromolecules and single molecules. A molecule which absorbs stronglyin the visible region of wavelengths is, for example, the heme group inthe hemoglobin molecule. Absorption causes visible light to lose part ofits intensity on the light path through the tissue. The absorptionmeasurements of the invention relate primarily to hemoglobin, the redblood pigment, which is also the strongest absorber in the visiblewavelength region in virtually all tissues.

In contrast to this, light loses no energy through elastic scattering.The incident electromagnetic wave which interacts with the scatteringcenter radiates the energy again, after excitation by the incident lightwave, into the various spatial angles. During this, the light retainsits wavelength, and only the direction of propagation of the light isaltered by the scattering centers. At the molecular level, the physicalprocess is to be conceived as excitation of the scattering particles bythe incident light wave and thereafter emission of the energy back intospace at the same wavelength. The directions in which the light energyis radiated then depend on the geometry, the shape and theelectromagnetic distribution of the electron shells of the molecule, andon the structure.

The incident, directed beam is converted on the path through the tissueinto diffuse radiation and adds up to forward scattering of the excitedscattering centers. After scattering, which is in some circumstancesmultiple, a small part of the incident light intensity returns to thesurface. The radiation backwards from all the scattering centers adds upto the backscattering. The light guides on the organ surface can detectonly light which has been backscattered and not quenched by absorptionon its light path. However, the only light detected by the light guideon the organ surface is that backscattered within the light guideaperture. This conception of the propagation of light is formallydescribed in the radiation transport theory.

It is an object of the invention to determine by measurement techniquesthe hemoglobin oxygenation and the hemoglobin concentration, andmeasurements derived therefrom, and to propose an instrument formeasuring the oxygen content and other related parameters (see Table 1).

This object is achieved according to the invention by an apparatushaving the features of claim 1. Dependent claims relate to embodimentsof the invention. A suitable name for the instrument according to theinvention would be AbTisSpec, an acronym for absorption tissuespectrophotometer.

Thus, the invention proposes an apparatus which determines the oxygencontent and data derived therefrom using an optical sensor to be placedon the tissue. It comprises one or more light sources which send lightthrough optical fibers to the sensor, one or more detectors whichreceive light backscattered by the tissue through optical fibers, and anevaluation unit which obtains information about the oxygen content fromthe backscattered light. An apparatus of this type is described forother purposes for example in EP 771 546 A2, the whole of the disclosureand technical design of instruments of which is here made subject-matterof the invention.

The light source preferably used is a white light source and/or a lasersource. Whereas the white light source introduces the light into thetissue whose backscattered portions later undergo spectral analysis, thelaser light source introduces a monochromatic light whose backscatteredportion has a measurable frequency shift so that a Doppler measurementand thus a rate measurement are possible.

A white light source (or various broad-band LEDs) is used for theillumination. A light source with a high illuminated field density and aspectrum of maximum whiteness and smoothness is crucial. The lightbackscattered from the tissue is spectrally analyzed by a polychromator,amplified and subsequently used as wavelength-dependent light intensitypattern for the evaluation.

The wavelength range from 500 to 650 nm (VIS) is particularly suitablefor measurements close to the surface. By contrast, the wavelength rangefrom 600 nm to 900 nm (NIR) is particularly suitable for depth-selectivemeasurements even at relatively large depths because the effective depthof penetration of the light in the NIR wavelength range is greater thanin the aforementioned wavelength range. In addition, different measuredvolumes are specified by the sensor geometry through different spacingsof illumination and detection regions. The combination of the selectionof detection spacings and appropriate wavelength ranges can thus beclearly defined and permits clear demarcation of individual measuredvolumes from one another. For calculation of the saturation values thereis evaluation of the spectral shape of the spectra, for example by ashape recognition and mixing method or of an approximation of thediffusion equation. The measurements obtained according to the inventionare, depth-selectively, the saturation of the hemoglobin with O₂ in thecapillary venous tissue bed.

The measured volume is determined by a novel method, namely by measuringan intensity gradient on the surface in combination with determinationof the absorption and scattering coefficients from the Dopplermeasurements, and of the spectrometric measurements.

The amount of hemoglobin can be obtained from the evaluation of thespectrometric data. The absorption caused by the attenuation of light bythe hemoglobin is determined by a method whose bases are described forthe first time by A. Krug, thesis, University of Erlangen-Nuremberg1998.

A large number of measured quantities is required to describe thecomplete oxygen supply situation (see Table 1). The arterial saturationand, independently thereof, the capillary venous saturation of thehemoglobin must be determined. It is possible from the differencebetween the arterial saturation and the saturation of the hemoglobin inthe capillaries, venules and veins then to determine the O₂ turnover orthe oxygen uptake. Combination of this differential amount with theblood flow values affords the 0, consumption in the investigated tissuevolume, which can thus be determined locally.

In summary, the following determination quantities must be measured todetermine the local oxygen supply:

-   1. Depth-selectively the saturation of hemoglobin SO₂ [%] in the    arterial system.-   2. Depth-selectively the saturation of hemoglobin SO₂ [%]; in the    capillary venous system.-   3. Depth-selectively the amount of hemoglobin in the measured    volume, from the absorption coefficients.-   4. Size of the measured volume, resulting from the surface gradient    measurement, the scattering coefficients and the anisotropy factors,    or from models of the diffusion theory.-   5. Depth-selectively the amount of blood flowing and the blood flow    rate.-   6. The local tissue temperature.

The combination of these measured quantities and the detection by anintegrating sensor which guarantees measurements in the same areapermits measurement of the local oxygen turnover of perfused tissue.

It is possible to provide as evaluation unit a spectrometer, aspectroscope, a laser Doppler spectroscope, a tissue spectrometer, atissue spectroscope, a pulse oximeter and/or a temperature gauge, eachalone or in any desired combination.

It is thus possible to determine

-   a) the local oxygen content,-   b) the local oxygen consumption in arterial/venous mixed tissue,-   c) the oxygen consumption rate in arterial/venous mixed tissue,-   d) the total amount of blood,-   e) the local oxygen transport capacity,-   f) the locally transported amount of oxygen,-   g) the oxygen turnover in arterial/venous mixed tissue,-   h) the oxygen turnover rate in arterial/venous mixed tissue and/or-   i) the local tissue oxygen partial pressure in perfused tissues.

In one embodiment of the invention, information is obtained fromdifferent depths by the selection of the wavelength range and of thedetector-transmitter separation. Thus, measurements can be obtained bothfrom the tissue surface and from deeper regions.

The apparatus according to the invention is suitable not only fordetection of hemoglobin and of the measurements which can be derivedtherefrom, but also for measuring the content of tissue pigments such ascytochromes, myoglobin, melanin, bilirubin or other pigments present inthe tissue, and data derived therefrom. The same optical sensor can beused for placing on the tissue for this purpose. Likewise the one ormore light sources which send the light through optical fibers to thesensor. In addition, one or more detectors which receive lightbackscattered from the tissue through optical fibers and pass it to anevaluation unit. The evaluation unit processes the received dataappropriately and measures the pigment content, its distribution or itstransport.

The light source which can be provided in this apparatus is also a whitelight source and/or a laser source.

It is likewise possible to provide as evaluation unit a spectrometer, aspectroscope, a laser Doppler spectroscope, a tissue spectrometer, atissue spectroscope, a pulse oximeter and/or a temperature sensor, eachsingly or in any desired combination.

Information from different depths can be obtained by the selection ofthe wavelength range and of the detector-transmitter separation.

In another embodiment of the invention, a bundle of optical fibers whichextends from the sensor to the detector or to a camera, such as a CCDcamera, can be provided so that a two-dimensional image of the recordedmeasurements can be generated. This makes it possible to produce anextremely vivid image of the two-dimensional hemoglobin distributionand/or of the oxygen saturation and/or of the oxygen parameters, asshown in Table 1, and/or of the distribution of other pigments in or onthe margin of a tissue.

On use of an additionally depth-selective sensor or of a depth-selectiveevaluation it is possible to generate a three-dimensional image of therecorded measurements. This permits a “view” into the interior of anorgan or of a tissue and permits layer-wise representation of therelevant data.

Crucial for these optical measurements according to the invention isdirect contact of the sensor with the organ surface. Only this makes itpossible to carry out measurements of backscattered light instead of,for example, measurements of reflected light. Only when the sensor is incontact is it possible to carry out depth-selective measurements or elsemeasurements related to the measured volume. For measurements of themicrocirculation it is, of course, crucial to construct appropriatemechanical applicators which do not impair the microcirculation even inthe capillary layer near the surface.

Further advantages, features and applications of the present inventionare evident from the following description of a number of exemplaryembodiments on the basis of the drawings. In this connection, all thefeatures shown belong separately or in any combination to the invention,irrespective of their inclusion in the description or the claims.

They are shown in:

FIG. 1 an integrated sensor,

FIG. 2 application form of the sensor, integration of laser Doppler andtissue spectrometer,

FIG. 3 application form of the sensor, integration of the laser Dopplerand tissue spectrometer,

FIG. 4 application form of the sensor, integration of tissuespectrometer and/or laser Doppler,

FIG. 5 application form of the sensor, integration of tissuespectrometer and/or laser Doppler,

FIG. 6 application form of the sensor, integration of the laser Dopplerand tissue spectrometer,

FIG. 7 a tissue spectrometer,

FIG. 8 a bilayer model,

FIG. 9 crude tissue spectra,

FIG. 10 hemoglobin spectra, 0 to 100% oxygenated,

FIG. 11 an ear sensor,

FIG. 12 a secure sensor,

FIG. 13 the extinction as a function of the hemoglobin concentration andoxygenation,

FIG. 14 the extinction as a function of the hemoglobin concentration intissue perfused in isolation,

FIG. 15 backscattering functions in the x and z directions

FIG. 16 a scattering cuvette,

FIG. 17 detection depths,

FIG. 18 transfer function of the x to z gradients,

FIG. 19 the model of a measured volume,

FIG. 20 two absorption spectra,

FIG. 21 an apparatus for recording two-dimensional measurementdistributions,

FIG. 22 a two-dimensional measurement distribution,

FIG. 23 an apparatus for recording three-dimensional measurementdistributions and

FIG. 24 cytochrome spectra, oxidized cytochromes and reduced cytochromesmeasured in a suspension of mytochondria.

FIG. 1 shows diagrammatically, viewed from below, that is to saystarting from the tissue, one embodiment, merely by way of example, ofan integrated sensor S or measuring head as can be used in the apparatusaccording to the invention. In this case it contains for theillumination the white light source W, the laser light source L and aplurality of detectors, where DD designates the detectors for Dopplersignals and DR designates the detectors for backscattered intensities.The linear arrangement shown is merely by way of example. Many otherarrangements are possible, with the detectors DR, which are more remotefrom the light source W, receiving light which has penetrated intodeeper regions of the tissue and has been backscattered there, asdescribed in EP 771 546 A2. A temperature sensor DT is additionallyshown.

According to the invention, photons from a coherent monochromatic lightsource L and, preferably, additionally photons from one or morebroad-band white light sources W are beamed through the integratedsensor system S into the tissue in a first region. The re-emergingphotons are detected (DD, DR) at various distances from this firstregion. In spatial alternation there is detection of the light for thelaser Doppler evaluation and for the spectroscopic or spectrometricevaluation. The design of the integrated sensor is depicted by way ofexample in FIG. 1.

The depth-selective laser Doppler measurements can be carried out withan apparatus as described in EP 771 546 A2.

According to the invention, FIG. 2 shows a specific form of theapplicator used in particular for endoscopic measurements in thegastrointestinal region. The advantage of this fiber arrangement is thecompact construction and easily reproducible geometry. As above, Wrepresents the point of illumination with a white light source, and Lrepresents the illumination with the laser source. In spatialalternation, the light is detected for the laser Doppler evaluation DDand for the spectroscopic or spectrometric evaluation DR. This sensorallows measurements in two detection depths in each case. Thetemperature sensor DT can optionally be arranged in the center of thesensor, or another fiber can.

FIG. 3 shows an arrangement from FIG. 2 which has been slightly modifiedaccording to the invention for applications with two separations for thelaser Doppler measurements DD and evaluation DR of the spectrometricand/or spectroscopic measurements in one separation, but with largertotal cross sections, because of the inclusion of the intensities ofthree detection fibers. FIG. 2 shows a white light source W and a laserlight source L.

The fiber arrangement in FIG. 4 shows an apparatus according to theinvention for determining the primary signals, obtained through a tissuespectrometer, of the arterial oxygen saturation, of the local oxygensaturation of the hemoglobin, and of the local hemoglobin concentration,as well as the parameters from Table 3. This arrangement has, because ofthe homogenous illumination of the central area by the six white lightsources W, which are shown here by way of example, particular advantagesof also being employable as reflection sensor. This apparatus ischaracterized in that the fibers W for illumination are located in acircle around the detector fiber DR. Light is transported into thesefibers W via one or more light sources to the sensor S.

FIG. 5 shows a form, modified according to the invention, of theapparatus shown in FIG. 4 for the same applications mentioned therein,but for measurements with greater depths of penetration. The greaterdepths of penetration are achieved because the transmitter-detectorseparation is larger. It is also possible to combine the two apparatuses(FIG. 5 and FIG. 4) if the illumination sources are used alternately inrelation to time or wavelength on a circular arrangement.

FIG. 6 is a modified form of the basis sensor from FIG. 1, in such a waythat in each separation a whole line consisting of at least 2 fibers wasarranged in place of one fiber in each case. This sensor permitsintegrating measurements in larger areas. The depth selectivity of thesensor from FIG. 1 is virtually retained.

FIG. 7 represents in principle the construction of a tissue photometeror tissue spectrometer. The core components are a broad-band lightsource W, optical fibers leading to the sensor S for illuminating thetissue, and optical fibers leading away from the sensor S for spectrallyanalyzed detection of the backscattered light from the tissue. Thedetection unit comprises a polychromator which both spectrally analyzesthe light and quantifies in a wavelength-dependent manner the detectedintensities. Thus, the tissue photometer makes available as initialvalues the color spectra which are subsequently used for the specificevaluations of the spectral information subsequent. In the design shownin FIG. 7 there is also a spectroscopic receiving unit which isconnected in parallel with the polychromator and which makes it possibleto pick up the detected backscattered intensities in a restrictedwavelength range or at single wavelengths with greater speed and highersensitivity. This detector unit is important for evaluating thepulsatile blood signals, for example for determining the arterialsaturation of hemoglobin.

A white light source W (or various broad-band LEDs) for example are usedfor the illumination. A light source with high illuminated field densityand a spectrum of maximum whiteness and smoothness is crucial. The lightbackscattered from the tissue is—spectrally analyzed by a polychromator—amplified and subsequently used as wavelength-dependent light intensitypattern for the evaluation.

The detector units necessary for detecting the hemoglobin levels nearthe surface have particularly high sensitivity in the visible wavelengthrange. The hemoglobin absorption values allow the hemoglobin levels tobe determined to a maximum depth of 4 millimeters in the wavelength bandfrom 500 to 650 nm.

According to the invention, two wavelength ranges are defined anddistinguished.

The wavelength range from 500 to 650 nm (VIS) is particularly suitablefor measurements near the surface, and the wavelength range from 580 to900 nm (NIR) is particularly suitable for depth-selective measurementsalso at greater depths and in larger volumes, because the effectivedepth of penetration of the light in the NIR wavelength range is greaterthan in the aforementioned wavelength range. A detector unit which hasparticularly high sensitivity in the wavelength range near the infraredis necessary for detecting hemoglobin levels in the macrovolume. Thehemoglobin absorption values in the wavelength range from 600 to 900 nmare a factor of 10 to 20 lower than in the visible wavelength range.Greater effective depths of penetration into the tissue are possible inprinciple owing to the reduced attenuation of light. In addition,different measured volumes are specified by the sensor geometry throughdifferent spacings of illumination and detection regions. Thecombination of the selection of detection spacings and appropriatewavelength ranges can thus be clearly defined and permits cleardemarcation of individual measured volumes from one another.

Selective measurements near the surface are possible according to theinvention firstly owing to the combination of the selection of thewavelength range from 500 to 650 nm and of the light guide separation ofless than 2 mm. Secondly, measurements in the macrovolume and in greatdepths of detection are possible owing to the selection of thewavelength range from 650 to 900 nm together with light guideseparations of more than 2 mm.

The detector unit with polychromator is the core of the tissuespectrometer according to the invention. A highest possible quantumyield and, resulting therefrom, a high detection frequency are desired.The greatest demands on the speed of detection are made on applicationof the method in cardiology, because the fastest physiological reactionsare to be expected here. The saturation of hemoglobin SO₂ pulsates onthe myocardium with the heart rate. The critical SO₂ values are to beexpected at the end of systole because myocardial perfusion is greatlyrestricted during the contraction because of the high pressure in theventricle. The heart rate is about 1/sec, with a systolic contractionduration in the region of 100 ms. This results in a maximally necessaryscan frequency of 10 msec in order to be able to resolve this intervalsufficiently. All the other physiological processes in the human bodytake place correspondingly slower and can be detected with lower scanfrequencies. Another advantage of high scan rates is the increasingsecurity of operation, which ensure “blur-free” recordings.

The following table summarizes the oxygen parameters for describing thelocal supply situation which are possible with the integrated sensorconcept according to the invention. In the table, a cross has been putat the place at which a signal is required either from the tissuespectrometer values, the laser Doppler data, the pulse oximeter valuesor from the temperature values in order to be able to determinetherefrom the corresponding oxygen parameter. The definition of thelocal oxygen supply quantities mentioned are explained in more detailhereinafter.

TABLE 1 Compilation of the methods by which it is possible with use ofthe tissue spectrometer, the laser Doppler spectroscope, the pulseoximeter and/or a temperature gage to determine the clinically relevantblood and oxygen supply quantities on integration or partial integrationof the sensors in a combined measuring head and integrative dataanalysis. Tissue spectrometer Measured vol. or Laser DopplerPulsoximeter SO₂ intensity Blood Velocity SO₂ Temp. Oxygen parametercap.-ven. Hb_(conc) gradient Hc_(conc/vol) flow v Pulsatility arterial TOxygen content X X X Oxygen consumption X X X X X Total amount of bloodX X O₂ transport capacity X X Amount of O₂— transported X X X Oxygenconsumption rate X X X X Oxygen turnover X X X X X X Oxygen turnoverrate X X X X Tissue pO₂ X X

The various oxygen parameters are derived, and the formal relationshipsof the methods according to the invention are defined, hereinafter.

Measurement of the oxygen content in the blood requires the amount ofhemoglobin in the investigated tissue volume to be determined and thesaturation of the hemoglobin present to be determined. It is thereforeindispensable also to be able to determine quantitatively theilluminated tissue volume, because the amount of hemoglobin must berelated to the value for the volume. The size of the illuminated ormeasured volume is crucially determined by the sensor geometry and thebasic physical-optical parameters of the tissue, which are formulated inthe form of absorption coefficients μ_(a)(λ) and scattering coefficientsμ_(s)(λ) in the spectral range used of the light sources W and L. Therelationships mentioned are derived in formal relations in the form ofmathematical formulae stepwise below:

The O₂ content in the blood can thus be determined formally by thefollowing formula:O_(2 Content) =Hb _(Amount)·SO₂ H

Thus, the measurement problem is divided into two different tasks. Onthe one hand, determination of the amount of hemoglobin Hb_(amount) and,on the other hand, determination of the saturation of hemoglobin SO₂with oxygen. Hüfner's number H produces the relationship betweenhemoglobin content and maximum oxygen content. Calculation of the amountof hemoglobin is described in turn in a following paragraph.

The oxygen turnover or the oxygen consumption in the tissue can beformally described by:O_(2 Consumption)=O_(2 Constant arterial)−O_(2 Content venous)

On the assumption that the amount of blood arterially and venouslyremains the same on the basis of the continuity equation, the formulacan be simplified to:O_(2 Consumption) =H·(Hb _(Saturationmaterial) −Hb_(Saturationvenous))·Hb _(Amount(in measured volume))

It is known from the literature that the arterial blood volume in thetissue is typically less th an 5%. It is thus sufficient to determineonly the amount of hemoglobin in the capillary venous system withoutcausing large errors.

If a restriction is made, in place of determination of the amount of O₂which has been consumed, to the oxygen consumption rate, an indicator ofthe ratio of arterial to venous consumption, the following equationresults therefrom for the determination:O_(2 Consumption rate)=(SO_(2 arterial)−SO_(2 venous))·Hb _(conc.)

The Hb_(conc.) can be determined in the same manner as defined below. Itis evident from the explanations hereinafter that Hb_(conc.) isproportional to the extinction (or else the optical density) based onthe particular measured volume.

It is not always possible in the tissue to combine the measured arterialsaturation with the relevant capillary venous hemoglobin saturation.Strictly speaking, this is the case only when the continuity equation issatisfied in the appropriate tissue volume. It is thus important forarterial and capillary venous systems to be present in the measuredvolume under consideration and for the arterial pulse to be detectable.

However, it is possible to determine locally for every tissue volume aquantity which is to be referred to as the transported amount of oxygenor local oxygen transport capacity. These quantities can be measuredwith the novel integrated sensor concept.

The local oxygen transport capacity is determined by the number ofmoving erythrocytes locally present and their hemoglobin content, whichis expressed by the locally moving hemoglobin concentration Hb_(conc.)multiplied by Hüfner's number H and multiplied by the flow velocityv_(blood) of the erythrocytes in the area investigated. Laser Dopplerinstruments which provide a spectral resolution of the velocity of theerythrocytes also provide a signal which reflects the amount of themoving erythrocytes (amount_(erys, moving)).

Since laser Doppler instruments calculate a value which corresponds to arelative perfusion Blood Flow, it is also possible in an approximatesolution for relative values to employ the following equation. The bloodflow in this case indicates a measure of the number of movingerythrocytes multiplied by their speed and thus represents a measure ofa volumetric flow. The result is thus:$O_{2\quad{transport}\quad{{cap}.}} = \frac{{H \cdot {Hb}_{{conc}.} \cdot {Blood}}\quad{Flow}}{{Amount}_{{Erys},{moving}}}$

The locally transported amount of oxygen is determined by the number oferythrocytes locally present multiplied by the flow velocity of theerythrocytes. This product is calculated by the laser Dopplerinstruments as a value which is referred to as blood flow. Blood flowmultiplied by Hüfner's number and the local oxygen saturation ofhemoglobin results in the transported amount of oxygen in the areainvestigated.Transported O_(2 Amount) _(rel.) =H·SO₂·Blood Flow

The oxygen turnover is proportional to the amount of O₂ which isconsumed in a defined area of tissue. The consumed amount of O₂ resultsfrom the difference between the amount of oxygen transported into thetissue (arterially) from the amount transported out again on the venousside. The quantitative oxygen turnover can be calculated as shown from:$O_{2_{turnover}} = {\left( {{SO}_{2_{arterial}} - {SO}_{2_{venous}}} \right) \cdot \frac{H \cdot {Hb}_{Amount} \cdot v_{Blood} \cdot {Amount}_{{Erys},{moving}}}{{in}\quad{the}\quad{measured}\quad{volume}}}$

The same assumptions, approximations and abbreviations already assumedabove apply in this case.

Since laser Doppler instruments calculate a value which corresponds to arelative perfusion Blood Flow, it is also possible in an approximatesolution for relative values to employ the following equation.O_(2 Turnover rate)=(SO₂ _(aeterial) −SO₂ _(venous) )·H·Blood Flow

A description of the calculation of the primary information follows:

For calculation of the hemoglobin saturations SO₂ it is possible, forexample, to analyze the spectral shape of the spectra by a shaperecognition and mixing method. The measurements obtained are thesaturation of hemoglobin with O₂ in the capillary venous tissue bed(described in W. Dümmler, Thesis, University of Erlangen, 1998).

In the present specific task of determining Hb spectra by tissuemeasurements, the absorption A is represented as total formed from thefundamental absorption A_(o) and the combined absorption portions of 0%and 100% oxygenated hemoglobin. The spectral absorption coefficients areexpressed by the specific extinctions of oxygenated Hb, ε_(ox) ^(Hb) anddeoxygenated extinction coefficients of Hb, ε_(deox) ^(Hb) in thefollowing equation:A(λ)=A _(o) +C _(ox)·ε_(ox) ^(Hb)(λ)+C _(deox)·ε_(deox) ^(Hb)(λ)

The coefficients C_(ox) and C_(deox) indicate the combination portionfrom which each measured spectrum can be composed according to itsdegree of oxygenation.

The scattering S is approximated in a first approach as a first orderwavelength-dependent function consisting of the linear combination offundamental scattering S_(o) and wavelength-dependent scattering portionS₁.S(λ)=S _(o) +λ·S ₁

The method described above is used to bring the measured spectra to theform A/S and equate them to the model approach, see right-hand side ofthe following equation.$\frac{A}{S} = \frac{A + {C_{ox} \cdot {ɛ_{ox}^{Hb}(\lambda)}} + {C_{deox} \cdot {ɛ_{deox}^{Hb}(\lambda)}}}{S_{0} + {\lambda \cdot S_{1}}}$

The hemoglobin saturation is determined by iterative determination ofthe coefficients$\frac{A_{o}}{S_{o}},\frac{C_{ox}}{S_{o}},\frac{C_{deox}}{S_{o}},\frac{S_{1}}{S_{o}}$by Newton's and the least squares methods and subsequent quotientformation: ${SO}_{2} = \frac{C_{ox}}{C_{ox} + C_{deox}}$

The hemoglobin saturation can thus lie only in the range of values from0% to 100%. The accuracy of calculation depends on the quality of thetissue model. The tissue model presented above can be extended at anytime, and it is thus possible to replace the basic absorption A_(o) bytissue-specific basic spectra A_(Tissue) (λ) which can be taken from anorgan table.

At the start of each measurement, in order to improve the spectrometervalues a dark spectrum should be recorded in order to establish theelectronic zero of the amplifier and the level of extraneous lightincident on the detector unit. It is necessary secondly to record aspectrum over a white standard in order to be able to establish theinstrument function of the lamp, of the sensor and of the completedetector unit. Depending on the quality of the spectrometer, thespectral accuracy should be carried out at defined time intervals by aspectral control measurement, for example using a mercury argoncalibration source. The described balance spectra should preferably berecorded with an average rate which is at least 10 times high er thanthe subsequent tissue spectra, because the errors in the dark spectrumand in the white standard spectrum are passed on through the spectralpreprocessing to all the measured data.

The recorded crude spectra must be preprocessed before they can be usedfor the evaluation. The backscattered spectrum R(λ) is composed of:${R(\lambda)} = \frac{{{Spectrum}_{crude}(\lambda)} - {{Spectrum}_{dark}(\lambda)}}{{{Spectrum}_{{white}\quad{standard}}(\lambda)} - {{Spectrum}_{dark}(\lambda)}}$

For this purpose, the spectrometer must run through a calibrationroutine, during which the dark spectrum and the white standard spectrumare recorded. The preprocessing of the spectra eliminates the chromaticerrors of the optical system of the instrument in accordance with theprior art.

To calculate the hemoglobin oxygenation in the VIS and NIR region,completely oxygenated hemoglobin spectra ε_(ox) ^(Hb)(λ) and completelydeoxygenated hemoglobin spectra ε_(deox) ^(Hb)(λ) are required. Thespectra should be recorded with the same wavelength resolution withwhich the measured spectra are also digitized. The tissue modelpresented by W. Dümmler (Thesis, University of Erlangen, 1998) can beextended according to the invention so that the specific organ spectraA_(tissue)(λ) are included directly in the model.${\frac{A}{S}(\lambda)} = \frac{{A_{tissue}(\lambda)} + {C_{ox} \cdot {ɛ_{ox}^{Hb}(\lambda)}} + {C_{deox} \cdot {ɛ_{deox}^{Hb}(\lambda)}}}{S_{0} + {\lambda \cdot S_{1}}}$

The specific tissue spectra can be obtained for each organ as a typicalaverage spectrum during hemoglobin-free perfusion.

It has been realized from many measurements that backscatteredhemoglobin spectra are distorted by erythrocytes and, in particular,compressed compared with spectra recorded in the transmissionconfiguration of light source and detector. The properties of theindividual spectrometer components play no part in this. The amplitudesof the hemoglobin spectra measured in the reflectance configurationshould be similar to the amplitudes of the Hb/HbO₂ reference spectra.This again means that suitable and comparable Hb/HbO₂ reference spectraare required in each case. The main reason for the compression of thespectra lies, according to the current state of knowledge, in thedifferences of the measured volumes of the various hemoglobinabsorptions. Light with wavelengths between 540 nm and 580 nmexperiences greater attenuation than in the adjoining wavelength rangesand therefore penetrates less deeply into the tissue. Light of the moreweakly absorbing wavelengths of the hemoglobin spectrum by contrastpenetrate more deeply into the tissue and therefore experience, inabsolute terms, a greater degree of attenuation than would be concludeddirectly from the extinction. It was possible to conclude thisrelationship directly from the measurements of the depths of penetrationin the bilayer models (see A. Krug, Thesis, Erlangen University, 1998).

A bilayer model which is composed of a scattering suspension (forexample Intralipid®, cells or tissue layers) disposed on top of a totalabsorber, such as, for example, marking ink, separated by a PVC filmonly a few μm thick, is described in FIG. 8. Two light guides extendinto the suspension; the first guides light in and the second picks upbackscattered light. The arrangement is used for determining the 90%depth of penetration, which will also be referred to as the detectiondepth.

Examples of results of measurements of detection depths in Intralipidsuspension are compiled in Table 2. The comparison shows that thedetection depths determined at 542 nm are far smaller than at 628 and760 nm, when hemoglobin modulates the measured volume because of itsdifferent absorption coefficients for the different wavelengths.

TABLE 2 Calculation of the detection depths, recorded with 200 μm quartzfibers in Intralipid suspension. Wavelength 542 nm 628 nm 760 nmDetection depths in μm for marking ink as background absorber IL 2% 9201000 1120 IL2% + 0.25 g 320 580 880 Hb/dl Detection depths in μm forblack PVC as background absorber IL 2% 1000 1040 1060 IL2% + 0.25 g 380660 920 Hb/dl

It is possible in principle to take two routes for solving the problemof distortion of the backscattered Hb spectra. Either the measuredspectra are rectified according to their hemoglobin concentration byrelating the extinctions to the effective measured volume in each case,or a whole series of HbO₂ reference spectra are generated with differenthemoglobin concentrations and different degrees of distortion and aremade available to the evaluation algorithm. The former solution is to bepreferred because, in this case, standard hemoglobin spectra fromcuvette measurements can be used, and the determination of the measuredvolume can additionally be used for quantitative calculation of thehemoglobin concentration.

The amount of hemoglobin is determined as described in the literature bythe following equation:Hb _(Amount) =C _(HB) ·V _(Meo)

It is clear from the above formula that to determine the amount ofhemoglobin it is necessary for the two quantities of the hemoglobinconcentration C_(Hb) (especially in the VIS) and the measured volumeV_(MEAS.) to be determined by measurement techniques and calculated.

Various theoretical approaches can be used to determine the amount ofhemoglobin in the particular measured volume. In the simplest case, thehemoglobin concentration is determined using the Lambert-Beer law (seenext formula):${{Ext}.} = {{\log\frac{I_{o}}{I}} = {C_{Hb} \cdot {ɛ_{Hb}(\lambda)} \cdot d}}$

The extinction Ext. is calculated from the logarithm of the lightintensity I_(o) beamed into the object, related to the light intensity Iemerging from the object. According to Lambert-Beer, the extinctiondepends on the hemoglobin concentration C_(Hb), the wavelength-dependentabsorption coefficient ε_(Hb) and the cuvette path length d.

The Hb_(conc.) is to be determined in the same manner as describedabove. Hb_(conc.) is proportional to the extinction (or else the opticaldensity) relative to the particular measured volume.${Hb}_{{conc}.} = {{K_{i} \cdot \frac{OD}{V_{{Meas}.}}} = {K_{i} \cdot \frac{\log\quad\frac{I_{o}}{I}}{ɛ_{Hb} \cdot V_{{Meas}.}}}}$

Determination of the measured volume is described hereinafter.

Another possibility for determining the hemoglobin concentration isprovided by the radiation transport equation. In its general form,however, no complete solution is possible and, for this reason, itcannot be manipulated particularly well. Therefore the diffusionapproximation of the radiation transport equation is frequently used intissue spectrometry.

The equations derived on the basis of the diffusion approximation areintroduced below. Using the diffusion approximation, an x gradient inthe reflectance configuration of the light guides is formally describedas:${I(x)} = {\frac{3P_{o}}{16\pi^{2}}\frac{\exp\left( {{- x}\sqrt{3{\mu_{o}\left( {\mu_{o} + \mu_{s}^{\prime}} \right)}}} \right)}{x}\left( {\mu_{o} + \mu_{s}^{\prime}} \right)}$

From this it is possible by condensing to two coefficients C₁ and C₂$\begin{matrix}{C_{1} = {\frac{3P_{o}}{16\pi^{2}}\left( {\mu_{a} + \mu_{s}^{\prime}} \right)}} \\{= {\frac{3P_{o}}{16\pi^{2}}\left( {{\rho\quad\sigma_{a}} + {\rho\quad{\sigma_{s}\left( {1 - g} \right)}}} \right)}}\end{matrix}$ and $\begin{matrix}{C_{2} = \sqrt{3{\mu_{a}\left( {\mu_{a} + \mu_{s}^{\prime}} \right)}}} \\{= \sqrt{3\rho\quad{\sigma_{a}\left( {{\rho\quad\sigma_{a}} + {\rho\quad{\sigma_{s}\left( {1 - g} \right)}}} \right)}}} \\{= {\rho \cdot \sqrt{3{\sigma_{a}\left( {\sigma_{a} + {\sigma_{s}\left( {1 - g} \right)}} \right)}}}}\end{matrix}$to condense the description of the gradients to:${I(x)} = {C_{1}\frac{\exp\left( {{- C_{2}}x} \right)}{x}}$

From the coefficients C₁ and C₂ found it is possible subsequently todetermine μ_(a) and μ_(s). The coefficient μ_(a)(λ) represents theabsorption coefficient of the tissue, from which it is possible, withappropriate approximations or after neglecting other absorbers in thetissue, to determine the hemoglobin concentration in the tissue.

Quantitative determination of the local hemoglobin concentration in themicrovolume by evaluating the backscattered spectra is a complex task.Various research groups have investigated the optical properties ofvarious tissues. It has emerged that the scattering coefficient μ_(s) isat least 10 times larger than the absorption coefficient μ_(a).Accordingly, the backscattered amount of light is primarily determinedby the scattering properties of the tissue investigated.

FIG. 9 shows three spectra, namely the spectrum of the instrumentfunction I_(inst.)(λ), the spectrum of an ideal scatterer I′_(o)(λ_(i))and a measured hemoglobin spectrum I_(m)(λ_(i)).

The two statements which follow are important for developing thesubsequent determination method for calculating the hemoglobinconcentration:

-   1. The only light intensities which can be measured on the tissue    surface are those reflected back by scattering in the tissue.-   2. If there is an absorber such as hemoglobin in the tissue it    attenuates the light on the path into the tissue between the    scattering events and on the path back to the detector light guide.

It is possible to conclude from the two statements that, at a wavelengthat which the absorption is negligibly small, the measured lightintensity is determined only by the backscattering. At all otherwavelengths where the absorption is not negligible, the light isattenuated by the absorber, and the intensity is therefore less than theundisturbed backscattered intensity.

Thus, a novel evaluation method for determining the Hb amplitudes oftissue spectra has been developed according to the invention. The firstpart has already been published in the thesis by A. Krug (1998, ErlangenUniversity). The method in which the intermediate results of the Hbdetermination are related, by extracting hemoglobin amplitudes from thebackscattered spectra, to the value for the measured volume, describedby a second method, at the same measurement point is novel. This resultsaccording to the invention in hemoglobin concentrations which arecontinually related to the current measured volume. The intermediatevalue of the relative hemoglobin concentration can also be found fromsolutions of the diffusion equation.

FIG. 9 depicts uncorrected spectra. The curve I_(inst.)(λ_(i)) shows theinstrument function or the optical error function of the tissuespectrometer. All measured spectra must be corrected by comparison withthis spectrum in order to eliminate the instrument-specificfalsifications of the measured spectra.

The curve I′₀(λ_(i)) corresponds to the spectrum obtained with a whitescatterer. The curve I_(m)(λ_(i)) corresponds to an actually measuredspectrum obtained with a physiological hemoglobin concentration in thescattering medium.

If the pure backscattered intensity I′_(o)(λ_(i)) is known, theproportion of light attenuation by the absorber can be found from thedifference between the pure backscattered intensity and the measuredintensity I_(m)(λ_(i)). The formal relationship can be expressed byI′ ₀(λ_(i))=I _(m)(λ_(i))+ΔI _(Abs)(λ_(i))

The unattenuated backscattered intensity I′_(o) is obtained inbackscattering photometry if the absorber concentration in the tissueequals zero. That is to say if only scatterers are present in thetissue.

FIG. 10 shows a hemoglobin spectrum I_(m)(λ_(i)) with oxygenation valuesfrom 0 to 100%, calculated by the color mixing method for determiningthe areas of the hemoglobin amplitudes. The assumption that a wavelengthexists, at which the absorption by hemoglobin is negligibly small existsat wavelengths greater than 640 nm. The wavelengths of the absoluteminima of the hemoglobin extinctions would be even more suitable, whichare at 690 nm for oxygenated and at 850 nm for deoxygenated hemoglobin.

FIG. 10 illustrates that the difference between I′₀(λ_(i)) andI_(m)(λ_(i)) corresponds to the absorption of light at this wavelength.Thus, the quotientlog[I′ ₀(λ_(i))/I _(m)(λ_(i))]also corresponds to the absorption. If this quotient is found, itrepresents a quantitative measure of the extinction or of the absorptionof light in this ideally scattering medium.

To calculate the hemoglobin concentration, the area of the extractedhemoglobin amplitudes is integrated, and an absorption is calculatedtherefrom for each spectrum. The integrated for values of oxygenated anddeoxygenated spectra differ, however. For this reason, in the secondstage of development, an oxygenation-dependent correction of theextracted hemoglobin amplitudes or of the area absorptions wasintroduced. The basic idea of this correction is to establish the areasof the differently oxygenated spectra and to carry out a correction forthe different levels of oxygenation. It was found that the area of afully oxygenated Hb spectrum, for example in the wavelength range500-630 nm is 16% larger than the area of a fully deoxygenated spectrum(see FIG. 10). The spectra in the literature, of Assendelft, 1970, wereused to establish these corrections.

It was also possible, by calculations with a color mixing method, toestablish the areas of all the intermediate levels of Hb oxygenationbetween the values of 0% and 100% HbO₂. The area of the tissue spectrais found after they are determined in a normalizing step independent ofthe level of hemoglobin oxygenation.

FIG. 11 shows an application of the optical oxygen sensor according tothe invention together with a temperature sensor as ear sensor. FIG. 11shows the auditory canal into which this special sensor head has beenintroduced, with eardrum. It can, for hygienic reasons, be provided witha transparent protective film. This hygienic cap and the sensor have aspecial shape adapted to the auditory canal and a type of mechanicalstop which prevents perforation of the eardrum. The length of the sensorup to the mechanical stop is about 25 mm. An optical oxygen sensor asshown in FIG. 1 to FIG. 6 or FIG. 21 or else FIG. 23 is present in thesensor head. Measurement of the oxygen saturation in the eardrum, aplanar structure, is crucial in this case. The combination with atemperature sensor provides the physician with the information herequires.

The temperature measurement methods which are suitable and preferredhere are non-contact methods such as infrared temperature measurement,or the methods for temperature measurement in the space of a closedauditory canal or the inner ear by NTC or similar temperaturemeasurement methods.

Since in this application form the optical oxygen sensor measures onlyin the membrane of the eardrum, which can be considered to be atwo-dimensional structure, it is possible in this specific case to carryout a reflection measurement in place of a backscattering measurement aswith measurements directly on organ surfaces. For this reason,non-contact infrared temperature measurement is to be preferred to anNTC sensor.

For optical measurements of the perfusion parameters also in amicrovolume it is important to develop appropriate applicators which, onthe one hand, guarantee that the sensor lies on and is thus in directcontact with the tissue and, on the other hand, permit a determinationwhen the contact pressure on the tissue is too high. The contactpressure must not exceed a certain level because, otherwise, theperfusion of the capillaries near the surface is impaired by thepressure of the sensor and thus may lead in some circumstances to anincorrect measurement.

Suitable temperature sensors are non-contact methods such as infraredtemperature measurement, or methods for temperature measurement in thespace of the closed outer ear or NTC or similar temperature measurementmethods.

In the embodiment of the applicator shown in FIG. 11, only reflectionmeasurements are possible because the applicator is not in contact withthe eardrum. Single-channel measurements with the tissue spectrometer,with the pulsatile tissue spectrometer, with the pulse oximeter, withthe laser Doppler and/or the temperature probe are sufficient forreflection measurements.

For imaging tissue parameters on the eardrum, in accordance with FIG. 21or FIG. 23, the ear stopper must be extended and must be placed directlyon the eardrum. For 2D and, in particular, 3D imaging it is againnecessary to carry out backscattering measurements in place ofreflection measurements.

FIG. 12 shows an embodiment of a novel sensor head. For this, a sensorhead as shown in FIG. 1 to 6, FIG. 21 or FIG. 23 is additionallyintegrated into a unit which makes it possible to detect simultaneouslywhether the sensor is reliably in contact with the tissue and whetherthe contact pressure of the sensor is not too high, and thus representsa pressure indicator.

This unit can be referred to as a secure sensor applicator. It comprisesa groove which is 1 to 2 mm wide and in which two pairs of light guidesare opposite one another on each side of the groove. On one side twotransmitting fibers, on the other side two detecting fibers. Thetransmitted light from each transmitting fiber is preferablyamplitude-modulated with a different frequency in each case. Thisamplitude modulation makes the secure sensor applicator secure againstinterference by foreign light sources and, moreover, permits separationof the light intensities coming from the two transmitting fibers. If thesensor is applied with the correct pressure, only a light attenuation ismeasured in channel 1, whereas no attenuation is yet detectable inchannel 2. If the tissue is deformed too much due to the contactpressure being too high, this leads to increased bulging of the skin orof the tissue surface into the groove of the sensor, leading to channel2 also then experiencing an attenuation of light transmission. It thussignals that the application pressure is too high and serves toestablish that the measurement conditions are impermissible and wouldlead to reduced capillary perfusion.

FIG. 13 shows the calculated relative extinctions (O.D.) at 5 differentHb concentrations.

FIG. 14 shows the functional relationship between the hemoglobinconcentration in a suspension and the calculated average relativeextinctions (O. D.).

The two FIGS. 13 and 14 summarize the results of an experiment anddocument the validity of the method presented. The results indicate therelationship between Hb concentrations pipetted into the suspension andthe oxygenation-dependent corrected Hb extinctions. The accuracy of theoxygenation-dependent correction for all hemoglobin concentration levelscan be read off from FIG. 13 for changes in oxygenation from 09 to 100%HbO₂. The largest difference can be read off at 1.0 g/dl.

The method for calculating the relative hemoglobin concentrationHb_(conc) in the NIR is to be designed in a similar way to thatdeveloped for the visible wavelength range. However, account must betaken of the particular spectral characteristics in the NIR. Theselection of the wavelength range considered is crucial for thecalculations with the Fitt algorithm because the Fitt algorithm can beapplied only if there is an appropriately characteristic spectraldifference depending on the hemoglobin oxygenation. It has emerged thatthe wavelength range 600-900 nm is particularly suitable formeasurements in the NIR wavelength range.

FIG. 15 shows the dependence of the backscattered intensity on the onehand on the transmitter-detector separation (x gradient) and on thedepth of penetration (z gradient). The x gradient of the backscatteredintensities as a function of the transmitter-detector separation and thez gradient can be determined in a scattering cuvette as shown in FIG. 16without marking ink.

The effective depth of penetration, also referred to as detection depthherein, can be determined by the unit shown in FIG. 16 (see A. Krug,Thesis, Erlangen University, 1998).

FIG. 16 shows a scattering cuvette filled with scattering let suspensionand marking ink for definition with depiction of the scanningdirections. A new light guide separation z was set with a micrometerscrew in each case and was then scanned in the x direction. The scanrepresents the light intensities of the various distances from the lightguides to the marking ink—the “black hole”.

FIG. 17 shows a survey picture of calculations of detection depths usinga 90% threshold in various Intralipid suspensions and with variousseparations, evaluated at 760 nm.

Now to the description of the calculation, which is novel according tothe invention, of the current measured volume in the VIS and in the NIR:the measured volume V_(meas.) can be determined from the measurement ofa surface intensity gradient. In addition, a transfer function whichmust be determined experimentally for the particular tissue is required.

As is to be seen in FIG. 18, the transfer function produces therelationship between the intensity gradient on the surface, which can beestablished by measurement techniques, and the unmeasurable intensitygradient in the depth of the tissue. The effective measurement volume isregarded as being the volume in which the backscattered intensitygradients have been 90% attenuated, as shown in FIG. 15. The transferfunction can be formally described by:I _(z gradient)(z)=transfer function⁻¹ *I _(x gradient)(x)

FIG. 19 shows the definition of the determination quantities a, b and cof a semiellipse to illustrate the measured volume formed by a lightwaveguide on illumination of a scattering tissue.

The measured volume can be determined approximately according to theinvention from the determination of the detection depths in the x and zdirection (see FIG. 15).

Assuming the measured volumes are semielliptical, the effective measuredvolume emerges according to FIG. 19 as:$V_{{Meas}.\quad{eff}.} = {\frac{2}{3}\pi\quad{abc}}$

The determination quantities of the ellipse a, b and c can be found bycalculating the effective depths of penetration from the intensitygradients. Because of the rotational symmetry of the illumination it ispossible to seta=b=x _(eff),the effective depth of penetration x_(eff) in the lateral direction. Thedepth c can be determined from the effective depth of penetration in thetransverse direction, andc=Z _(eff)is determined via the effective depth of penetration in the z direction,which is to be directed perpendicularly into the tissue. The 3parameters of the semielliptical volume integral and thus the relevantmeasured volume are thus determined.

In the study by A. Krug [Krug, Thesis, Erlangen-Nuremberg University,1998] it was proposed that the measured volume decreases as the absorberconcentration increases. It is possible to show by correcting themeasured backscattered extinctions by the reduced measured volume ineach case that a linear relationship between the amount of absorber inthe scattering suspension and the extinction relative to the currentmeasured volume is obtained even with measurements in highly scatteringmedia.

A method for determining the arterial oxygen saturation by means of abroad-band tissue spectrometer including all the spectral information isproposed according to the invention.

In a first approach, the arterial saturation of hemoglobin is determinedby the usual pulse oximeter method with evaluation of at least twowavelengths, there being formation of a pulse-synchronous differencesignal of the heart beat. These wavelengths should preferably beselected so that depth-selective determination of these arterial valuesis also possible, and so that the outputs of the monochromatic and ofthe broad-band light source are effective in an additive manner at thesewavelengths. In a first approach, the conventional pulse oximeter methodis included in the integrated sensor head.

A novel approach according to the invention is to determine the arterialsaturation from the broad-band backscattered spectra of the tissuephotometer. The tissue photometer allows the current hemoglobinsaturation in the measured volume to be calculated via the methoddescribed above for determining the hemoglobin saturation. It ispossible to detect the pulse-synchronous changes in saturation via aparticularly rapid photometer whose scanning times in the region of 1-10ms per value. The tissue photometer always detects an averageappropriate for the volume-mixing ratio of arterial and capillary venoussaturations.

According to the invention the arterial pulse is detected by laserDoppler signal evaluation and is used to trigger the tissuespectrometer.

The systolic blood pressure results in an increase in the blood flow andthe blood volume in the tissue. In accordance with the theory of pulseoximetry, “fresh”, completely saturated arterial blood is pushed intothe tissue for this systolic increase in blood volume. This results alsoin total in a greater saturation of the blood in the measured volume ofthe tissue photometer. If, for the systolic blood volume addition, thereis no determination of the diastolic blood volume and the saturationduring systole and diastole it is possible with the rearranged mixingequation to determine the arterial saturations.

SO_(2 Mix.syst.) ·Hb _(Amount) _(Mix.syst.) =SO_(2 art.) ·ΔHb _(Amount)_(art.) +SO_(2 Diast.) ·Hb _(Amount) _(Diast.)${SO}_{2{art}} = \frac{{{SO}_{2\quad{{Syst}.}} \cdot {Hb}_{{Amount}_{{Syst}.}}} - {{SO}_{2\quad{{Diast}.}} \cdot {Hb}_{{Amount}_{{Diast}.}}}}{\Delta\quad{Hb}_{{Amount}_{{art}.}}}$

FIG. 20 shows two exemplary spectra. The 80% saturated spectrumcorresponds to a state at the end of systole when the content of freshoxygen-rich blood is greater than at the time of the period of theslower, diastolic perfusion during which the saturation essentiallycorresponds to the capillary venous saturation.

The blood volume ΔHb_(amount, art.) is formed from the difference in theamounts of blood and hemoglobin at the end-systolic and end-diastolictime point. The method for hemoglobin determination as described aboveis used in this case too.

The saturations SO_(2 syst.) and SO_(2 diast.) are determined, likewiseas described above, by evaluation of the curves of the tissue spectra.

However, the pulse oximeter method should also be determined from thespectrometric data set as pulse-synchronous difference signal in orderto be able to draw valid conclusions owing to the greatly expandedspectral database. It is of particular interest to evaluate thedifference signal of the spectrometric data because this differencesignal can be related to the capillary venous baseline value and thus itis possible for the first time to determine a quantitative arterialoxygen saturation (see FIG. 20).

FIG. 20 shows two absorption spectra and their difference signal duringthe pulse-synchronous change in the spectra on the assumption ofconstant hemoglobin concentrations in the measured volume.

In another embodiment of the invention, a two-dimensional and athree-dimensional imaging of the local oxygen parameters (as shown inTable 1) by an imaging method is presented:

The description of the invention until now has related to pointmeasurements in the vicinity of the illumination source, which consistseither of a laser source, of LEDs or of a white light source.

However, in medicine, many imaging methods are in demand, starting withX-ray films, ultrasonic images and on to magnetic resonance images,which are very easily accessible to the trained eye of the physician.Information summarized in the form of images additionally provides anexcellent possibility for transmitting a large amount of informationwith a high degree of order. A method for recording initiallytwo-dimensional and subsequently three-dimensional images of the localdistribution of the oxygen parameters which can be measured with thesensor described above is presented here.

The sensor technique for recording the various local oxygen parametersas listed in Table 1 has been explained in the preceding sections and isto serve as basis here. Differing from the sensor technique describedabove with point detection of the oxygen parameters, a novel, primarilytwo-dimensional imaging method is explained below.

FIG. 21 shows the design in principle of the recording apparatusaccording to the invention for two-dimensional imaging of oxygenparameters. In this variant, the core of the sensor consists of animaging light guide (also called imaging bundle) as already used inendoscopes or catheters in order to pass image information from theinterior of the body to the outside. The imaging light guide is composedof a bundle of single fibers which are, however, arranged so that theimage information is retained. This imaging light guide is arrangedbetween the instrument and the measurement point on the object. In themeasuring instrument, this imaging light guide is then scanned with thepoint measuring probe placed, as described above, directly on theobject. The immense advantage of this arrangement derives from the factthat all the moving parts can be attached in the measuring instrument,and the sensor, in this case the end of the imaging light guide, can befixed directly on the surface to be measured. The novel imaging sensorneeds to be fixed only once. The scanning in the instrument means thatthe object is not displaced by the scanner movement itself, and it isnow only necessary to scan the flat surface on the instrument side ofthe imaging light guide. This surface can be scanned with greater speedbecause the mechanical conditions therefor can be defined better in theinstrument.

It is crucial that the individual fiber diameters in the imaging lightguide are less than or equal to the cross sections of the pointmeasuring probe. This ensures that the same sensor geometry is alwaysproduced by the imaging light guide as previously also defined directlyby the point sensor. The imaging of the point sensor increases inaccuracy as the density of the packing of the fibers increases and thethickness of the individual fibers of the imaging light guide decreases.The second alternative, exact 1:1 coupling of the individual fibers ofthe point sensor in each case into exactly one individual fiber of theimaging light guide is also possible but far more complicated.

It is also possible according to the invention to use a plurality ofpoint sensors at the same time. This requires a multichannelillumination source and a multichannel detection unit. This makes itpossible to reduce the time for a complete image because, if the pointsensors are arranged side by side, for example in the y direction, itwould then be necessary to carry out only half the number of scans inthe x direction.

As an alternative to the imaging light guide, or as replacement for ascanner and the complete detection unit, it is also possible to useother cameras such as CCD cameras with an additional unit, which makedirect spectral analysis of the detected information possible. Theseadditions to the cameras have not to date made adequate spectralresolution possible, but this might soon be realized with furthertechnical developments.

FIG. 22 shows the image of a scanned perfused liver surface, whichdemonstrates the local distribution of the hemoglobin saturation. Inthis method, which is still very elaborate, the point sensor was stilladvanced directly to the liver surface. Uncoupling of the movement ofthe point sensor from the organ surface was achieved by a PVC filmstretched in between. The image is that of the distribution of thehemoglobin saturation on an isolated perfused liver surface. Inaccordance with the compilation of optical methods in Table 1, it isalso possible to obtain images of the other oxygen parameters and theother tissue pigments mentioned in Table 3.

It emerged from these experiments that the sensor geometry is crucialfor the spatial resolution with which the distribution of the oxygenparameters can be imaged. An appropriate sensor resolution appropriatefor the morphological structure of the tissue which is to beinvestigated should therefore be selected via the fiber diameter,aperture and fiber material.

A three-dimensional recording method for imaging local oxygen parametersis built up on the two-dimensional recording method. As shown in FIG.23, a similar type of imaging light guide (also called imaging bundle)as depicted in FIG. 21 is used. However, in contrast thereto the imaginglight guide is now according to the invention scanned in the instrumentwith the depth-selective sensor from FIG. 1 with an x-y scanner. Thetwo-dimensional imaging is combined with the depth-selective recordingof the oxygen parameters via the sensor with a plurality of channelsdiffering in separation both for the Doppler measurements and for thedetection of the backscattered intensities.

One embodiment of the invention also permits determination of othertissue parameters with the spectrometric measurement methods describedabove:

Besides the chromophore hemoglobin, it is of interest to determine byspectrometry other pigments (summarized in Table 3) which occur intissue, such as, for example, cytochromes, myoglobin, melanin andbilirubin.

Under physiological tissue conditions, measurements of other pigments atthe same time as hemoglobin are very difficult because the absorptioncaused by the hemoglobin completely obliterates the spectra of the otherpigments mentioned. However, determination of the tissue pigmentsmentioned in addition to hemoglobin is of physiological and clinicalinterest. Determination of the tissue pigments mentioned is possible inorgans perfused free of hemoglobin or in pathologically alteredsituations. It is possible according to the invention to measure theredox state of the cytochromes, for example during organtransplantation, in the hemoglobin-free state. It is also possible toinvestigate the myoglobin oxygen saturation and concentration inskeletal and heart muscle.

The methods for producing 2-dimensional and 3-dimensional images of theoxygen parameters shown in Table 1 and/or of the derived parametersshown in Table 3, obtained from tissue levels through combination of thesignals from the tissue spectrometer and/or the laser Doppler and/or thepulse oximeter and/or temperature sensor are novel, especially in theircombination of the methods.

The importance of monitoring these tissue substances is that this opensup the possibility of being able to investigate directly theintracellular oxygen supply conditions. The Fitt algorithm describedabove can be converted to calculation of the redox state of thecytochromes investigated by providing completely reduced and completelyoxidized cytochrome reference spectra (see FIG. 24). FIG. 24 showscytochrome spectra, oxidized cytochromes and reduced cytochromes,measured in mitochondrial suspension.

It is possible in the same way, by providing completely oxygenated andcompletely deoxygenated myoglobin spectra, also to calculate themyoglobin saturation with oxygen via the Fitt algorithm described above.

The spectral method according to the invention for determining thehemoglobin concentration from backscattered spectra can be applied in asimilar way for the tissue pigments mentioned herein for determining theintracellular cytochrome, myoglobin, melanin and bilirubinconcentrations.

The wavelength range from 500 to 650 nm is particularly suitable fordetermining the cytochrome and myoglobin levels with abovementionedmethods because, in this wavelength range, the naturally low absorptioncoefficients of these cellular absorbers have the comparatively highestvalues. It is thus possible in this wavelength range to attain theclearest absorption spectra with the best signal-to-noise ratio.

Melanin, the skin pigment, and bilirubin, a hemoglobin breakdownproduct, have less characteristic spectral curve shapes than do thecytochromes. Myoglobin and hemoglobin can therefore be measuredquantitatively only less specifically and less unambiguously.

The method for determining concentrations of an absorber present in thetissue can, of course, also be extended to artificially injected dyes.It is thus possible for color influx and efflux curves to be measurednoninvasively and locally in the tissue directly.

TABLE 3 Pigments and/or pigment concentrations which naturally occur inthe tissue and/or are introduced into the tissue and which can bedetermined with the tissue spectrometer using the methods according tothe invention Pigment Determination parameter Method Hemoglobin Oxygensaturation Fitt algorithm, with reference spectra Hemoglobin Areaintegration concentration method with tissue normalization CytochromesRedox state Fitt algorithm, with reference spectra Intracellular Areaintegration cytochrome method with tissue concentration normalizationMyoglobin Myoglobin oxygenation Fitt algorithm, with reference spectraIntracellular myoglobin Area integration concentration method withtissue normalization Melanin Melanin concentration Absorptiondetermination Bilirubin Bilirubin concentration Absorption determinationArtificial Specific dye Absorption dyes concentration determinationPermeability tests Color efflux method

It is thus possible and preferable to carry out the followingmeasurements and calculations with the apparatus according to theinvention:

To determine the oxygen content, only the output signals of thebroad-band tissue backscattering spectrometer are evaluated. The oxygensaturation (SO₂) is determined from the tissue spectra via the colorinformation, and the hemoglobin concentration (Hb_(conc.)) is determinedfrom the attenuation of light. The tissue levels can be measureddepth-selectively by the multichannel tissue spectrometer signalsmeasuring at various separations, and be related to the relevantmeasured volume, which is determined on line. Multichannel spectrometermeasurement is crucial because this measuring configuration allows theSO₂ values and/or the Hb_(conc.) values to be related to the measuredvolume. The measured volume is determined by gradient measurements andsubsequent determination of absorption and scattering. Measurements atthe same site and time through the same sensor (see FIG. 1) are crucialin order to be able to ensure relation of the signals to the particularmeasured volumes.

To determine the oxygen consumption in arterial/venous mixed tissue, theoutput signals of the tissue spectrometer are in evaluated together withthe pulsatile signals either from a pulse oximeter or from a fast tissuespectrometer. In the case of pulsatile tissue spectrometer measurement,the largest saturation values are evaluated differentially during acardiac cycle in order to be able to discriminate only the arterialblood contribution. The high data acquisition rate is crucial in orderto be able to detect the arterial circulation pulse. Withoutdiscrimination of the pulse it is impossible to determine any arterialO₂ saturation. Triggering of the measurement time point for the arterialdetermination is defined from the laser Doppler signals and/or thetime-resolved signals of the Hb_(conc.) determination. This innovativecalculation method for determining the arterial saturation is based onevaluation of the signals from a fast broad-band tissue spectrometer(<20 ms per clock period) and/or the triggering via the signal of thelaser Doppler unit.

Measurements at the same site and time through the same sensor (seeFIG. 1) are crucial in order to be able to ensure relation of thesignals to the particular measured volumes.

The total amount of blood, also referred to as the total amount oftissue hemoglobin, is determined by evaluation of the multichanneltissue spectrometer signals which are carried out at the same time atdifferent separations. The hemoglobin concentration relative to themeasured volume is determined from the broad-band tissue backscatteringspectra. The total amount of blood in the measured volume is establishedtherefrom by including the laboratory parameters of the hematocrit andthe mean corpuscular hemoglobin.

The oxygen transport capacity is determined from the output signals fromthe multichannel tissue spectrometer measurement and the hemoglobinconcentration signal, together with the blood flow output signal of thelaser Doppler method. Measurements at the same site and time through thesame sensor (see FIG. 1) are crucial in order to be able to ensurerelation of the signals to the particular measured volumes. It isimportant to employ the two methods together in order to be able todetermine the oxygen transport capacity. The blood flow on its own isinsufficient to allow any statement about the hemoglobin content of theerythrocytes and thus also about the oxygen binding and transportcapacity. The hemoglobin concentration on its own in turn provides noinformation about the movement or velocity of the erythrocytes.

To determine the locally transported amount of oxygen, once againmultichannel output signals from the tissue spectrometer, the oxygensaturation signal, the hemoglobin concentration, the output signals fromthe laser Doppler and the blood flow rate or the blood flow must becombined together in order to achieve maximum accuracy. Determination atthe same site and time of the tissue spectrometer signals and the laserDoppler signals through the same probe in measured volumes which arethus comparable is crucial for obtaining these signals. Measurements atthe same site and time through the same sensor (see FIG. 1) are crucialin order to be able to ensure relation of the signals to the particularmeasured volumes.

The oxygen consumption rate in arterial/venous mixed tissue is arelative numerical measure dispensing with an absolute relation of theHb_(conc) and SO₂ to the measured volume. It is therefore possible inthis case also to carry out single channel spectrometer and pulseoximeter measurements. The output signals of the tissue spectrometer(SO₂ and Hb_(conc)) and the pulse oximeter signals or the pulsatiledifferential output signals of the tissue spectrometer (arterial SO₂)are combined and evaluated synchronously. Measurements at the same siteand time through the same sensor (see FIG. 1) are crucial in order to beable to ensure relation of the signals to the particular measuredvolumes.

To determine the oxygen turnover, the primary signals of the broad-bandtissue spectrometer (SO₂ and Hb_(amount)) of the pulsatile tissuespectrometer, of the pulse oximeter (SO_(2 art.)) and/or the laserDoppler primary signals (v_(blood), Amount_(erys, moving)) are evaluatedin a multichannel manner in order to be able to achieve the relation tothe measured volume and/or the depth selectivity of the values. Theoxygen turnover indicates the difference in the volumetric flow of O₂delivered arterially and transported away venously. Measurements at thesame site and time through the same sensor (see FIG. 1) are crucial inorder to be able to ensure relation of the signals to the particularmeasured volumes. In addition, determination of the measured volumethrough spectral intensity gradients is crucial for relating thenumerical measure of amount to the joint measured volume.

To determine the oxygen turnover rate in arterial/venous mixed tissue,the quantitative relation to the measured volume is dispensed with. Alsonecessary for this purpose are single channel or multichannel tissuespectrometer signals (SO₂) and single channel or multichannel pulsatiletissue spectrometer signals and/or pulse oximeter signals (SO_(2 art.))and single channel or multichannel laser Doppler signals (blood flow)for calculating the oxygen turnover rate.

To determine the local tissue oxygen partial pressure (pO₂) it isnecessary to determine at the same site and time the primary signals ofthe tissue spectrometer (SO₂) and the temperature (T), and thelaboratory parameters (pCO₂ and 2, 3 BPG). The local PO₂ can beapproximately determined for capillary venous and/or arterially suppliedtissue by discrimination of capillary venous SO₂ and arterialSO_(2 art.).

The method for determining the local hemoglobin concentration fromtissue spectra is also according to the invention. This is normalized tothe approximated basic tissue spectrum as shown in FIG. 9 with, at thesame time, the Hb_(amount), which is calculated from the extracted Hbamplitudes, being related to the relevant measured volume of the sensor.The measured volume was determined from light intensity gradients whichare evaluated spectrally in order to determine therefrom the transferfunction as shown in FIG. 24 and/or the absorption and scatteringcoefficients of all the wavelengths involved from the diffusion theory,to which the Hb_(amount) value is related.

Methods for determining the arterial oxygen saturation by differentialevaluation of the time-resolved spectral backscattered signals as shownin FIG. 20 with the aid of a fast broad-band tissue spectrometry arealso according to the invention. It is also possible to combine with alaser Doppler system for triggering the arterial flow pulse time pointfor determining the oxygen saturation with the largest, that is to sayarterial, saturation contribution. The advantage of this combination isthat the spectrum recording rate of the tissue spectrometer nownecessary is thus only relatively low.

1. An apparatus for ascertaining the local oxygen turnover and/or theoxygen consumption and/or the O₂, transport capacity and/or thetransported O₂ amount and/or the oxygen consumption rate and/or theoxygen turnover rate and/or the oxygen turnover rate and/or data derivedfrom the content of tissue pigments, ascertained from the primarysignals of the local hemoglobin concentration and/or the content oftissue pigments and/or the local oxygen saturation and/or the arterialoxygen saturation and/or the blood flow rate and/or the transportedamount of blood and/or the tissue temperature with an optical sensor (S)for placing on the tissue, characterized by at least one white lightsource (W) and at least one laser source (L) which send light to thesensor (S), one or more detectors (DD, DR) which receive lightbackscattered from the tissue, and an evaluation unit, and characterizedin that optical fibers are provided between light sources (W, L) andsensor (S) and between sensor (S) and detectors (DD, DR), with theoptical fibers of the sensor (S) preferably being arranged on a circularshape around a central fiber or a temperature probe (DT), and in thatone fiber each for the white light source (W) and for the laser (L), andin each case at least two detection fibers (DR, DD) lie on an arc of acircle at defined distances from the illumination sources, each of whichis fed to a separate evaluation.
 2. The apparatus as claimed in claim 1,characterized in that a spectrometer, a spectroscope, a laser Dopplerspectroscope, a tissue spectrometer, a tissue spectroscope and/or apulse oximeter and/or a temperature measurement (DT) ore provided asevaluation unit.
 3. The apparatus as claimed in claim 1, characterizedin that the primary signals are related to a optically determinedmeasured volume and/or in that the measured volume of the optical sensoris determined and information is obtained from various depths byevaluation of the various wavelength ranges and at least onedetector-transmitter separation.
 4. The apparatus as claimed in claim 1,characterised in that the detection fibers (DR) are evaluated together.5. The apparatus as claimed in claim 1, characterized by a bundle ofoptical fibers which extends from the sensor (S) to the detector or to acamera, such as a color CCD camera, so that a two-dimensional imago ofthe evaluated signals can be generated.
 6. An oxygen sensor as set forthin claim 1 for measurements on the eardrum, in which the primary signalsof the tissue spectrometer (SO₂, HB_(amount))) and/or of the pulsatiletissue spectrometer and/or of the pulse oximeter (SO_(2 art)) and/or ofthe laser Doppler (blood flow) are recorded in a reflection measurementand combined with one another in order to be able to determine theoxygen parameters and/or the pigment parameters via the ear sensor. 7.The apparatus as claimed in claim 1, characterized in that the fiberswith a separation x_(i) are illuminated and/or evaluated together. 8.The apparatus as claimed in claim 1, characterized in that a pressureindicator signal in generated via opposing light guides and/or lightexit and entry regions and indicates the deformation of the tissueand/or of a membrane because of application of the sensor.
 9. Theapparatus for ascertaining the local oxygen turnover and/or the oxygenconsumption and/or the O₂ transport capacity and/or the transported O₂amount and/or the oxygen consumption rate and/or the oxygen turnoverrate and/or the oxygen turnover rate and/or data derived from thecontent of tissue pigments, ascertained from the primary signals of thelocal hemoglobin concentration and/or the content of tissue pigmentsand/or the local oxygen saturation and/or the arterial oxygen saturationand/or the blood flow rate and/or the transported amount of blood and/arthe tissue temperature with an optical sensor (s) for placing on thetissue, characterized by at least one white light source (W) and atleast one laser source (L) which send light to the sensor (S), one ormore detectors (DD, DR) which receive light backscattered from thetissue, and an evaluation unit, and characterized in that theilluminated fibers for a white light source and/or a laser light sourcelie on an open or closed are of a circle directly around the centralfiber and are illuminated by one or more light sources, with detectionof the backscattered and/or laser Doppler signals taking place throughthe central fiber.
 10. The apparatus as claimed in claim 9,characterized in that the illuminated fibers (W) and/or (L) lie on alarger radius and/or on different radii of a circle which areilluminated synchronously and/or alternately.
 11. An apparatus forascertaining the local oxygen turnover and/or the oxygen consumptionand/or the O₂ transport capacity and/or the transported O₂ amount and/orthe oxygen consumption rate and/or the oxygen turnover rate and/or theoxygen turnover rate and/or data derived from the content of tissuepigments, ascertained from the primary signals of the local hemoglobinconcentration and/or the content of tissue pigments and/or the localoxygen saturation and/or the arterial oxygen saturation and/or the bloodflow rate sensor the transported amount of blood and/or the tissuetemperature with an optical sensor (S) for placing on the tissue,characterized by at least one white light source (W) and at least onelaser source (L) which send light to the sensor (S), one or moredetectors (DD, DR) which receive light backscattered from the tissuesand an evaluation unit, characterized by a bundle of optical fiber,which extends from the sensor (S) to the detector or to a camera, suchas a color CCD camera, so that a two-dimensional image of the evaluatedsignals can be generated, and characterized by an additionallydepth-selective sensor (S) or a depth-selective evaluation so that athree-dimensional image of the recorded measurements can be generated.